Okay, so.
1. The mine entrance room you go to on foot (28) is not the same room as the one in the unicorn cutscene -- that's entirely done in room 333, which pretends to be various other rooms. Hence the loading time when you ride horse -- it's preloading all these backgrounds.
2. The mine door is open or closed depending on the time of day you enter room 28, or when room 333 pretends to be 28.
3. The fake mine room (333 pretending to be 28) has the same door view (600) at the same positions as the real room 28, for both 1988 and 1989 versions.
4. There is nothing in this cutscene script that could put the door where it's shown in the video or screenshot.
(5. The mockery started during a lull in the research and wasn't directed at you.)
I'm gonna stop trusting the scripts for a bit, try to see this action for myself.
Edit: overtime nosleep, tp 27, tp 333, wait a bit and... there's nothing wrong with this picture at all. The door's open and in the right place. This is King's Quest IV, version 1.006.004, released 1989-08-07 on SCI 0.000.502.